1. Field of the Invention
The present invention relates to marine structures such as jackup rigs, jackup barges, jackup platforms, and the like and more particularly relates to an improved load transfer system for jackup rigs having a barge and a plurality of support legs which are vertically movable in order to raise the platform from a floating position on the surface of the ocean to a stable position with the legs at their lowermost end bearing on the seabed.
Even more particularly, the present invention relates to a shock absorbing, load monitoring and load transfer system using a substantially frictionless hydraulic component, which system has the capability of distributing the load within the overall platform support system making the loading configuration a statically determinate problem as opposed to a normally statically indeterminate case with platform having four or more legs, for example.
2. General Background
In the offshore oil and gas drilling industry, for example, and with oil/gas wells which have already been drilled and are producing, a method used to provide a barge or working platform above the well often involves the use of a barge which is raised above the surface of the water and above the flotation provided by the ocean to a level thereabove wherein three or more legs bear at their lower end portion on the seabed and thus support the rig barge or platform in a stable fashion thereabove.
There are numerous "jackup platforms" in use in the world, many for the above-mentioned offshore drilling or production purposes, and some for offshore well logging and maintenance and the like. Others are used for various other types of offshore work such as dredging, mining, harbor construction, and some are involved in bridge construction and/or civil engineering purposes. Many of these platforms have need for a load monitoring device which can tell the operators of the rig the respective loads being carried by each of the many legs which support it.
A load monitoring device using strain gauges is available on the market and is one of the systems which has been used consistently to monitor load on jackup rig legs.
The jacking mechanism of some platforms consists of hydraulic jacks. Such jacks are disclosed in various U.S. patents such as U.S. Pat. Nos. 2,540,679; and 4,040,265. With such hydraulic jacking mechanisms it is possible to have an idea of the weight of the platform during the jacking operation by monitoring the pressure on the hydraulic jacks. However, the mechanical friction between several components such as the piston-cylinder, piston rod-rod end bottom, legs-leg guides, prevents any accurate weighing of the platform during the jacking operation. Once the platform is jacked up, the jacking mechanism is locked for several reasons. It is rather dangerous to rely on the piston seals of these type jacking mechanisms to keep the platform in position for a very long period of time. Further, when the jacking mechanism is locked up it becomes impossible to determine the weight of the platform and the load distribution on the legs.
There are on most platforms large variation in weights and positions of the variable load. This is very critical on small cantilever type jackup drilling barges. Some owners of such platforms rely on computer calculations to determine the legs load but the computer calculations are only as good as the data they receive. Thus, if they receive wrong data the values for loads are useless.
Another type of jacking mechanism is provided with a rack and pinion arrangement. In this manner, the only way to estimate the jackup weight is to measure either the current in the electric systems (or the pressure delivered in the hydraulic motors) or the torque of one of the shafts in the mechanism driving the jacking operation. Again, due to friction, the results are inaccurate and the error is of the same order of magnitude as the variable load.
One system which has been patented in an attempt inter alia to approximate the weight of the platform or which could possibly be used to approximate the weight of a platform is described in U.S. Pat. No. 3,986,368 entitled "Load Equalizing and Shock Absorber System for Offshore Drilling Rigs" issued Oct. 19, 1976, to Clarence W. Levingston. However, this system cannot always be precise use of hydraulic cylinder assemblies and due to the friction of the piston and piston rods in these cylinders. Moreover, a leak in any of the cylinders can make the whole system inoperative. The presence of seals in these assemblies produces a source of failure upon wearing of the seal as can be enhanced by repetitive operation especially in the presence of shear loads.
3. General Discussion of the Present Invention
The present invention solves the prior art problems and shortcomings by providing a reliable, precise, robust load leg monitoring system which is also capable of improving the sharing of the load on the different legs of a platform. In addition, the system can reduce the impact load on a marine platform when the legs first touch bottom. Furthermore, the present invention provides a system which equalizes the load of the different jacking mechanisms of each of the legs. Also provided with the present invention is a short stroke jacking capacity.
It is thus an object of the present invention to provide a reliable, precise, robust leg load monitoring system for a jackup platform.
It is another object of the present invention to provide a load transfer and monitoring system for jackup legs which can determine a platform weight and the horizontal position of the center of gravity on the platform.
It is another object of the present invention to provide a means for improving the sharing of the load between the different legs of a jackup platform having four or more legs.
Another object of the present invention is to distribute evenly the load on one leg between several jacking mechanisms.
Still another object of the present invention is to provide a means of preloading the legs of the platform of a jackup rig having four or more legs without using ballast or adding any weight on the platform, thereby substantially reducing ballast space requirements.
Another important object of the present invention is to provide a jackup platform operator a single system to prevent overloading of one of the platform legs with all the possible consequences.
Another object of the present invention is to provide a load monitoring and transfer system for jackup rigs which absorbs shock transferred from the legs to the jacking structure and then the barge during raising or lowering of the legs.
Another object of the present invention is to provide a load monitoring and load transfer system for jackup rigs which is not disturbed with shear loads generated between the legs of the platform and the jacking structure.
Another object of the present invention is to provide a load monitoring and load transfer system for jackup rigs, which utilizes hydraulically actuated pads, and further allows continued operation of the apparatus when one of the pads becomes damaged by isolating it in the system.